Test for user functions.
Also test an <expr> mapping calling a function.
Also test that a builtin function cannot be replaced.

STARTTEST
:so small.vim
:function Table(title, ...)
:  let ret = a:title
:  let idx = 1
:  while idx <= a:0
:    exe "let ret = ret . a:" . idx
:    let idx = idx + 1
:  endwhile
:  return ret
:endfunction
:function Compute(n1, n2, divname)
:  if a:n2 == 0
:    return "fail"
:  endif
:  exe "let g:" . a:divname . " = ". a:n1 / a:n2
:  return "ok"
:endfunction
:func Expr1()
:  normal! v
:  return "111"
:endfunc
:func Expr2()
:  call search('XX', 'b')
:  return "222"
:endfunc
:func ListItem()
:  let g:counter += 1
:  return g:counter . '. '
:endfunc
:func ListReset()
:  let g:counter = 0
:  return ''
:endfunc
:func FuncWithRef(a)
:  unlet g:FuncRef
:  return a:a
:endfunc
:let g:FuncRef=function("FuncWithRef")
:let counter = 0
:inoremap <expr> ( ListItem()
:inoremap <expr> [ ListReset()
:imap <expr> + Expr1()
:imap <expr> * Expr2()
:let retval = "nop"
/^here
C=Table("xxx", 4, "asdf")
 =Compute(45, 0, "retval")
 =retval
 =Compute(45, 5, "retval")
 =retval
 =g:FuncRef(333)

XX+-XX
---*---
(one
(two
[(one again:call append(line('$'), max([1, 2, 3]))
:call extend(g:, {'max': function('min')})
:call append(line('$'), max([1, 2, 3]))
:$-7,$w! test.out
:delfunc Table
:delfunc Compute
:delfunc Expr1
:delfunc Expr2
:delfunc ListItem
:delfunc ListReset
:unlet retval counter
:q!
ENDTEST

here